The present invention relates generally to a game racket frame, and more particularly to a game racket frame made of a composite material.
The conventional game racket frame of the composite material is generally made by a method, in which the fibers impregnated with resin are formed into a composite material containing fibers which are arranged unidirectionally by a fiber-winding roller. The composite material so produced is then folded at various angles for being cut into various composite materials having different widths and lengths. The composite materials of various widths and lengths are stacked windingly to form a game racket frame and a triangular portion, which are subsequently arranged in a molding tool in which a game racket frame is made under heat and pressure. The game racket frame so made is characterized in that it has a better expansibility and a smooth surface, and that its frame and triangular portion are made integrally. However, the process of making such a prior art game racket frame as described above is not cost-effective in view of the fact that the manufacturing process is almost manually operated, and that the angle at which the composite material is folded and cut can not be varied easily, and further that a substantial amount of waste material is produced to complicate the environmental problems.
With a view to overcoming the shortcomings of the prior art method described above, a method known as the filament winding method was introduced. As shown in FIG. 1, the filament winding method involves the production of a tubular element 10, which is made of the fibers preimpregnated with resin by winding. The tubular element 10 is bent to have the form of a game racket frame 11 and a triangular throat portion 12, as shown in FIG. 2, and is then subject to a B-stage treatment before being arranged in the mold cavity 14 of a molding tool 13 provided with an air valve 15 through which the air is forced thereinto. The molding tool 13 is heated under pressure. Upon completion of a hardening process, the molding tool 13 in opened to remove therefrom a newly formed game racket frame of the composite material. The filament winding method can be automated to reduce the labor cost and to improve the production efficiency. In addition, the waste material produced in the filament winding method is reduced in quantity. Nevertheless, the game racket frame of the composite material and made by the filament winding method has several shortcomings, which are described explicitly hereinafter.
The triangular throat portion has a limited length, thereby resulting in a technical difficulty in implementing the filament winding method and in an increase in the production cost. In addition, the triangular throat portion made by the filament winding method can not be caused to cooperate well with the frame to form a good air blowing loop.
The construction of the game racket frame of the composite material made by the filament winding method is defective in design in that the fiber expansibility is unduly limited to result in the game racket frame being nonuniform in thickness, and that the gaps are formed among the wound fibers, thereby undermining the structural strength of the game racket frame. Although the structure of the game racket frame can be strengthen by means of increasing the layers of the wound fibers, it is not allowable to increase the layers of the wound fibers too much in view of the strict weight limit of the game racket frame.
The different spots of the game racket frame will suffer different stresses, in view of the different shapes and the changes of striking positions of the game racket frames. Consequently, it is required to reinforce the weak spots of the game racket frame. In theory, the requirement can be met by reinforcing the weak spots with additional wound fibers; nevertheless it is technically infeasible that such a reinforcement can often result in an excessive amount of the fibers 16 that are arranged at 90.degree. orientation in the area where the wound fibers are turned. In other words, such a reinforcement as described above can result in the concentration of stress and can even weaken the structure of the game racket frame. Furthermore, the reinforcement of the weak spots can be also done by a change in the winding angle of the additional wound fibers, however; in view of the strict weight limit of the game racket frame and the limit of the winding speed, such a reinforcement is technically feasible only with a tubular element having a relatively large diameter and not with a tubular element having a small diameter of the game racket frame. And, as shown in FIG. 3, if various local reinforcements are done within the same winding layer by a change in the winding angle in various specific areas, the changed winding angle must be greater than the original winding angle. Any winding angle that is greater than the original winding angle can undermine the structural strength of the reinforced areas.
The frame of the game racket is generally provided with a groove to facilitate the stringing. It is conceivable that the shock wave is transmitted to the string groove from the strung surface impacted by a ball. It is therefore necessary to have the string groove that is stronger structurally. The stress direction of the string must form a great angle with the string groove. There is a strict weight limit of the game racket frame, so it is technically infeasible to reinforce the string hole areas by the filament winding method in view of the fact that the reinforcement of the string groove will result in an excessive increase in the weight of the game racket frame. In addition, the reinforcement of the string groove by a winding near 0.degree. orientation, as indicated by the dotted lines in FIG. 4, will not result in a meaningful reinforcement of the structural strength of the string hole area of the game racket.